Systems and Methods for Indicating Buffered Data at an Access Point Using an Embedded Traffic Indication Map

ABSTRACT

Stations in standby mode periodically wake up to check for buffered data at the access points. Traditionally, the information is available by checking the periodic beacon frame for a traffic indication map (TIM). Unfortunately, the length of beacons has steadily increased with the progression of the various wireless standards requiring stations to wake up for longer periods to merely check for buffered data. Several approaches are disclosed which address this shortcoming, including the broadcast of TIM frames, the partial reception of beacon frames and the use of an embedded TIM frame within a beacon frame.

CROSS-REFERENCE TO RELATED APPLICATION

Under 35 U.S.C. 119, this application claims priority to, and thebenefit of, U.S. Provisional Patent Application entitled, “EmbeddedTraffic Indication Map,” having Ser. No. 60/948,047, filed on Jul. 5,2007, which is incorporated by reference in its entirety.

This application is also related to U.S. patent application entitled,“Systems and Methods for Indicating Buffered Data at an Access PointUsing a Traffic Indication Map Broadcast”, having Ser. No. 12/046,946,filed on Mar. 12, 2008, which claims priority to U.S. Provisional PatentApplication entitled, “TIM Broadcast,” having Ser. No. 60/906,608, filedon Mar. 13, 2007 and to U.S. Provisional Patent Application entitled,“Check Beacon Indication,” having Ser. No. 60/970,195, filed on Sep. 5,2007, which is incorporated by reference in its entirety.

This application is also related to U.S. patent application entitled,“Systems and Methods for Indicating Buffered Data at an Access Pointwith Efficient Beacon Handling”, having Ser. No. 12/047,021, filed onMar. 12, 2008, which claims priority to U.S. Provisional PatentApplication entitled, “Partial Beacon Reception,” having Ser. No.60/932,791, filed on May 31, 2007, which is also incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure generally relates to wireless communications andmore particularly relates to systems and methods for retrieving buffereddata from an access point.

2. Background Information

Among other things, FIG. 1 illustrates a typical network configurationfor communicating data between stations via an access point in awireless local area network (WLAN) or 802.11-based network. Asillustrated in the non-limiting example of FIG. 1, a network 140 may becoupled to access point 130. In some embodiments, the network 140 may bethe Internet, for example. Access point 130 can be configured to providewireless communications to various wireless devices or stations 110,120, 124. Depending on the particular configuration, the stations 110,120, 124 may be a personal computer (PC), a laptop computer, a mobilephone, a personal digital assistant (PDA), and/or other deviceconfigured for wirelessly sending and/or receiving data. Furthermore,access point 130 may be configured to provide a variety of wirelesscommunications services, including but not limited to: Wireless Fidelity(WIFI) services, Worldwide Interoperability for Microwave Access (WiMAX)services, and wireless session initiation protocol (SIP) services.Furthermore, the stations 110, 120, 124 may be configured for WIFIcommunications (including, but not limited to 802.11, 802.11b,802.11a/b, 802.11g, and/or 802.11n).

Access point 130 periodically broadcasts a beacon frame to variousstations at a beacon period. The beacon frame is used by an access pointto announce its presence and to relay information. For example, ifstation 110 is a laptop and is powered up or is transported to alocation within range of access point 130, station 110 listens for abeacon frame from all access points in its range. Each access pointwithin range transmits a beacon frame and depending on the system, theuser at station 110 can select which access point to use, thereby makingan association between station 110 and the access point.

In order to save power, stations can be put into standby mode. Whilesleeping, the access point buffers data intended for the station. Also,for the purposes of this disclosure the term “sleep mode” will be takento mean an operating state entered by a computing device either uponinitiation by a user or after expiration of a period of sufficientinactivity in which the amount of power supplied to the device isreduced as compared to the amount supplied during normal operation. Thestations in standby mode wake up to receive the beacon frame. Containedwithin the beacon frame is a traffic indication map (TIM) element whichindicates for which stations the access point has buffered data waiting.If the station has determined that the access point has buffered datafor it, the station can retrieve the data by sending a Power Saving-POLL(PS-Poll) frame.

FIG. 2 shows an example of the retrieval of buffered data by the stationusing a PS-Poll frame. In sequence 200, PS-Poll frame 202 sent by thestation is followed immediately after a short interframe space (SIFS) bythe data frame 206 sent by the access point. After another SIFS, thestation responds with acknowledgement (ACK) 210.

As illustrated in FIG. 3, the stations in standby mode wake up at thetarget beacon transmission time (TBTT) in order to receive the beaconframe. After a beacon frame is received, the station can determine theTBTT for the next beacon frame from the start time of the current beaconframe and the beacon interval value transmitted in the current beaconframe. In the timeline shown here, beacon frames 302, 304, 306, 308, and310 represent five beacon frames transmitted by the access point. Thefrequency of the beacon frames is represented by the period equal to thebeacon interval. Each of the beacon frames in the example contains a TIMelement; specifically, beacon frames 302, 304, 306, and 310 contain TIMelements 312, 314, 316, 318 and 320, respectively. Periodically, the TIMelement in the beacon frame is a delivery traffic indication map (DTIM),which indicates after the beacon frame the access point will transmitbuffered multicast or broadcast data.

Stations in standby mode, in addition to determining whether the accesspoint has any buffered data waiting for it, may also require additionalinformation from the beacon frame. For example, the access point mayannounce a channel switch through a channel switch announcementinformation element which can be included in a beacon frame or someother announcement mechanism such as a probe response as describedbelow. The channel switch announcement information element indicatesthat the Basic Service Set (BSS) will move to another channel shortly.This information is needed by the station so that it can follow thechannel change; otherwise, it will wake only to find that the beaconframe no longer is transmitting on the present channel.

In order for any station in standby mode, which has been associated withan access point to determine whether that access point has buffered datawaiting for that station, the station must periodically wake up toreceive the TIM element within the beacon frame. However, the length ofbeacon frames has grown over time with the progression of standards andimplementation of more and more features. This would require the stationto stay awake longer to receive a lengthy beacon frame, which can causea station to consume more power in standby mode. Furthermore, beaconframes are generally transmitted at a low physical layer (PHY) rate,often at the lowest PHY rate allowable. Because the transmission rate isso low, the station must stay awake longer to receive the beacon frame.This has an adverse effect on the battery life of handheld devices.Accordingly, various needs exist in the industry to address theaforementioned deficiencies and inadequacies.

SUMMARY OF INVENTION

In the past, TIMs have been used by access points to indicate tostations in standby mode that buffered traffic awaits it. The methodsdisclosed enable the reception of the TIM in a more efficient fashion bythe stations. Further features enable the stations to selectivelyreceive beacon frames which can unnecessarily increase the time astation in standby mode has to awaken.

An embedded TIM (ETIM) element can be included in a beacon frame nearthe beginning of the beacon frame, requiring the station to only receivepart of the beacon frame to determine if buffered data is waiting. TheETIM element can also comprise multiple partial virtual bitmaps when theTIM is sparse. The ETIM element need not be included in every beaconframe and can be included in DTIM beacon frames. Additionally, the ETIMelement includes a check beacon indication. The indication signifieswhether a critical or significant change has taken place in the beaconframe. The indication can be implemented by incrementing representativecounters when such changes take place. A station can then check theindication to decide whether to receive the remainder of the beacon.

Access points and stations comprising a processor, network interfacesand a memory can be configured to interoperate with the methods andvariations described above by implementing additional logical modules asinstructions in the memory. The logic can then be carried out by theprocessor.

Other systems, methods, features, and advantages of the presentdisclosure will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 illustrates a typical network configuration for communicatingdata between stations via an access point in a WLAN or 802.11-basednetwork;

FIG. 2 shows an example of the retrieval of buffered data by the stationusing a PS-Poll frame;

FIG. 3 illustrates the timing of stations in standby mode waking up atthe TBTT in order to receive a beacon frame;

FIG. 4 illustrates an embodiment of one of the wireless devices/stationsshown in FIG. 1;

FIG. 5 illustrates an embodiment of an access point shown in FIG. 1;

FIG. 6 shows the first eleven fields in the basic format of a beaconframe as used in various wireless standards in a recommended order;

FIG. 7 shows a TIM element within a beacon frame as used in variouswireless standards;

FIG. 8 shows a TIM control frame format;

FIG. 9 illustrates a modified TIM element for possible use in a TIMframe for broadcast;

FIG. 10 shows an embodiment of a TIM element having two partial virtualbitmaps;

FIG. 11 shows a TIM management action frame format;

FIG. 12A illustrates the timing where a TIM frame is transmitted priorto a TBTT;

FIG. 12B illustrates the timing where a TIM frame is transmitted at aTBTT;

FIG. 12C illustrates the timing where a TIM frame is transmitted at apredetermined offset relative to a TBTT;

FIG. 13A illustrates the timing where a TIM frame is transmitted priorto a DTIM TBTT;

FIG. 13B illustrates the timing where a TIM frame is transmitted at aDTIM TBTT;

FIG. 14A illustrates one timing for transmission of two TIM frames atdifferent PHY rates;

FIG. 14B shows a timeline where both a high rate TIM frame and a lowrate TIM frame are individually scheduled;

FIG. 15 shows multiple TIM frames sent as a burst;

FIG. 16 illustrates a TIM control frame which includes check beaconfield;

FIG. 17 illustrates the alternative embodiment of a TIM frame with acheck beacon indication where the TIM frame is a management actionframe;

FIG. 18 is a flowchart showing exemplary logic which can be implementedin the software of a station showing the interoperation of the receivingof the TIM, the check beacon indication, and receiving of the beacon;

FIG. 19 illustrates the first portion of a beacon frame in accordance toone embodiment of the invention;

FIG. 20 illustrates an exemplary embodiment of the check beaconinformational element;

FIG. 21 illustrates a partial frame check informational elementexemplary embodiment;

FIG. 22 shows an exemplary first portion of the beacon frame using anETIM element; and

FIG. 23 shows the format of an ETIM informational element.

DETAILED DESCRIPTION

A detailed description of embodiments of the present invention ispresented below. While the disclosure will be described in connectionwith these drawings, there is no intent to limit it to the embodiment orembodiments disclosed herein. On the contrary, the intent is to coverall alternatives, modifications and equivalents included within thespirit and scope of the disclosure as defined by the appended claims.

FIG. 4 illustrates an embodiment of one of the wireless devices/stationsshown in FIG. 1. It can be configured to receive and process messages asdisclosed below. Generally speaking, station 120 can comprise any one ofa wide variety of wireless computing devices, such as a desktopcomputer, portable computer, dedicated server computer, multiprocessorcomputing device, cellular telephone, PDA, handheld or pen basedcomputer, embedded appliance and so forth. Irrespective of its specificarrangement, station 120 can, for instance, comprise memory 412,processing device 402, a number of input/output interfaces 404, wirelessnetwork interface device 406, display 408, and mass storage 422, whereineach of these devices is connected across one or more data buses 410.Optionally, station 120 can also comprise a network interface device420, also connected across one or more data buses 410.

Processing device 402 can include any custom made or commerciallyavailable processor, a central processing unit (CPU) or an auxiliaryprocessor among several processors associated with the computing device120, a semiconductor based microprocessor (in the form of a microchip),a macroprocessor, one or more application specific integrated circuits(ASICs), a plurality of suitably configured digital logic gates, orgenerally any device for executing instructions.

Input/output interfaces 404 provide any number of interfaces for theinput and output of data. For example, where station 120 comprises a PC,these components may interface with user input device 404, which may bea keyboard or a mouse. Where station 120 comprises a handheld device(e.g., PDA, mobile telephone), these components may interface withfunction keys or buttons, a touch sensitive screen, a stylist, etc.Display 408 can comprise a computer monitor or a plasma screen for a PCor a liquid crystal display (LCD) on a hand held device, for example.

Wireless network interface device 406 and, optionally, network interfacedevice 420 comprise various components used to transmit and/or receivedata over a network environment. By way of example, these may include adevice that can communicate with both inputs and outputs, for instance,a modulator/demodulator (e.g., a modem), wireless (e.g., radio frequency(RF)) transceiver, a telephonic interface, a bridge, a router, networkcard, etc. Station 120 can use wireless network interface device 406 tocommunicate with access point 130.

With further reference to FIG. 4, memory 412 can include any one of acombination of volatile memory elements (e.g., random-access memory(RAM), such as DRAM, and SRAM, etc.) and nonvolatile memory elements(e.g., flash, read only memory (ROM), nonvolatile RAM, etc.). Massstorage 422 can also include nonvolatile memory elements (e.g., flash,hard drive, tape, CDROM, etc.). Memory 412 comprises software which mayinclude one or more separate programs, each of which includes an orderedlisting of executable instructions for implementing logical functions.Often, the executable code can be loaded from nonvolatile memoryelements including from components of memory 412 and mass storage 422.Specifically, the software can include native operating system 414, oneor more native applications, emulation systems, or emulated applicationsfor any of a variety of operating systems and/or emulated hardwareplatforms, emulated operating systems, etc. These may further includenetworking related software 416 which can further comprise acommunications protocol stack comprising a physical layer, a link layer,a network layer and a transport layer. Network related software 416 canbe used by processing device 402 to communicate with access point 130through wireless network interface 406 and can further include logicthat causes the station to wake up at a proscribed time to receive oneor more TIM frames, a TIM element or ETIM element, depending on theembodiment, to determine if access point 130 has buffered data for it.The software can further include logic which retrieves the buffered datausing a PS-Poll message if there is buffered data and checks the checkbeacon indication within the received TIM frames, beacon frame or ETIMelement, depending on the embodiment, to decide whether to receive theentire beacon frame. In particular, the software can receive a wakeupinstruction from the access point even in a protected wireless network.It should be noted, however, that the logic for performing theseprocesses can also be implemented in hardware or a combination ofsoftware and hardware. One of ordinary skill in the art will appreciatethat the memory 412 can, and typically will, comprise other componentswhich have been omitted for purposes of brevity.

FIG. 5 illustrates an embodiment of an access point shown in FIG. 1. Itcan be configured to receive and process messages as disclosed below.Generally speaking, station 120 can comprise any one of a wide varietyof network functions, including network address translation (NAT),routing, dynamic host configuration protocol (DHCP), domain nameservices (DNS) and firewall functions. Irrespective of its specificarrangement, the stations 120 can, for instance, comprise memory 512, aprocessing device 502, wireless network interface 504, network interface506, and nonvolatile storage 524, wherein each of these devices isconnected across one or more data buses 510.

Processing device 502 can include any custom made or commerciallyavailable processor, a CPU or an auxiliary processor among severalprocessors associated with access point 130, a semiconductor basedmicroprocessor (in the form of a microchip), a macroprocessor, one ormore ASICs, a plurality of suitably configured digital logic gates, orgenerally any device for executing instructions.

Wireless network interface device 504 and network interface device 506comprise various components used to transmit and/or receive data over anetwork environment. By way of example, either interface may include adevice that can communicate with both inputs and outputs, for instance,a modulator/demodulator (e.g., a modem), wireless (e.g., RF)transceiver, a telephonic interface, a bridge, a router, network card,etc.). Access point 130 typically uses wireless network interface device504 to communicate with nearby stations, and network interface device506 to communicate with network 140. In some implementation, the twodevices can be combined into one physical unit.

With further reference to FIG. 5, memory 512 can include any one of acombination of volatile memory elements (e.g., RAM, such as DRAM, andSRAM, etc.) and nonvolatile memory elements (e.g., flash, ROM,nonvolatile RAM, hard drive, tape, CDROM, etc.). Memory 512 comprisessoftware which may include one or more separate programs, each of whichincludes an ordered listing of executable instructions for implementinglogical functions. Often, the executable code and persistentconfiguration parameters can be loaded from nonvolatile memory elementsincluding from components of memory 512. Specifically, the software caninclude native operating system 514, one or more native applications,emulation systems, or emulated applications for any of a variety ofoperating systems and/or emulated hardware platforms, emulated operatingsystems, etc. These may further include networking related software 522which can further comprise a communications protocol stack comprising aphysical layer, a link layer, a network layer and a transport layer.These may further include networking related software 516 which canfurther comprise a communications protocol stack comprising a physicallayer, a link layer, a network layer and a transport layer. Networkrelated software 516 can be used by processing device 502 to communicatewith access point 130 through wireless network interface 506 and canfurther include logic that causes the access point to broadcast one ormore TIM frames, which can include a check beacon indication, at aproscribed time. Alternatively, the software can include logic thatcauses the access point to transmit a beacon frame with the TIM elementplaced near the beginning of the beacon frame and include a check beaconindication. The software can include logic that causes the access pointto transmit a beacon frame with an ETIM element placed near thebeginning of the beacon frame. In particular, the software can receive awakeup instruction from the access point even in a protected wirelessnetwork. It should be noted, however, that the logic for performingthese processes can also be implemented in hardware or a combination ofsoftware and hardware. One of ordinary skill in the art will appreciatethat the memory 512 can, and typically will, comprise other componentswhich have been omitted for purposes of brevity.

FIG. 6 shows the first eleven fields in the basic format of a beaconframe as used in various wireless standards in a recommended order.First is the timestamp field comprising the time the present frame issent. Following that is the beacon interval field which represents thenumber of time units between TBTTs, followed by the capabilityinformation field containing a number of subfields that are used toindicate requested or advertised capabilities. It should be noted thatthe first three fields are fixed length fields.

The remaining fields are information elements. The first of these in therecommended order is the service set identity (SSID) element whichindicates the identity of an extended service set (ESS) or independentbasic service set (IBSS). The supported rates element specifies thecommunications rates that are supported in accordance with a specificstandard. The frequency-hopping (FH) Parameter Set element contains theset of parameters necessary to allow synchronization for stations usinga FH physical layer and is only present when FH physical layers areused. Direct sequence (DS) parameter set element contains information toallow channel number identification for stations using a direct sequencespread spectrum (DSSS) physical layer. The coordination function (CF)parameter set element contains the set of parameters necessary tosupport the point coordination function (PCF). The IBSS Parameter Setelement contains the set of parameters necessary to support an IBSS(e.g., an ad hoc network). This is followed by the TIM element which isdescribed in further detail below. Country element indicates whichcountry the access point is in. Further detailed description of any ofthese fields is given in their individual standards.

FIG. 7 shows a TIM element within a beacon frame as used in variouswireless standards. The first octet is field 702 which contains theelement ID, a unique code used to identify the type of element inaccordance with the given wireless standard. For example, in 802.11, theelement ID, 5, is assigned to the TIM element. The second octet islength field 704 indicating length in octets of the remaining fields inthe element. The remaining fields are sometimes referred to as theinformation field. The next octet is DTIM count field 706 indicating howmany beacon frames, including the current beacon frame before the nextDTIM. The next octet is DTIM period field 708 indicating the number ofbeacon periods between DTIM. The next octet is bitmap control field 710comprising a plurality of bits indicative of various features of thebitmap to follow, including the offset into the bitmap. Partial virtualbitmap field 712 comprises 1 to 251 octets. Each bit in partial virtualbitmap 712 refers to a single station through a mapping of associationidentifiers (AIDs) to bits in partial virtual bitmap 712, where themapping is specified by the individual standard. The value of the bit isindicative of whether the associated station has buffered data waiting.

Rather than require a station in standby to wake up to receive acomplete beacon frame to determine if the access point has buffered datafor the station, a TIM frame containing the same information as the TIMelement within the beacon frame can be broadcast a proscribed time. Anystation in standby mode associated with the access point can wake up toreceive the TIM frame broadcast to determine whether there is anybuffered data waiting for it. Since the TIM frame is much shorter thanthe beacon frame, the station will be awake for a much shorter time andhence consume less power. The TIM frame is shorter because it containsless octets than a typical beacon, but also because it may betransmitted at a higher rate than the beacon. A TIM element could beincorporated into either a control frame or a management action frame,to form a TIM frame.

FIG. 8 shows a TIM control frame format. Frame control field 802 is atwo octet fixed field indicative of properties of the frame as definedby the particular standard. Duration/ID field 804 is a two octet fixedfield which comprises either duration information or identificationinformation depending on the frame use as defined by the particularstandard. Receiver address field 806 is a six octet fixed field whichcomprises an address indicative of the receiving station, but since thisis a broadcast, the special broadcast address as specified by theparticular standard is used here. Following receiver address field 806is TIM element 808 which can vary from 6 to 257 octets. Finally, framecheck sequence field 810 is a four octet fixed field indicative of theintegrity of the frame. The specific integrity check is specified by thestandard, but as an example, some standards use a cyclic redundancy code(CRC).

TIM element 808 can use the same format as TIM element described in FIG.7 However, DTIM count field 706 and DTIM period field 708 are notmeaningful unless the TIM element is in a beacon frame. Therefore, amodified TIM element as described below in FIG. 9 can also be used.

FIG. 9 illustrates a modified TIM element for possible use in a TIMframe for broadcast. The format for the modified TIM element isidentical to the format shown in FIG. 9 except DTIM count field 706 andDTIM period field 708 have been removed. If the TIM element of FIG. 9 isused, then the number of octets the TIM element occupies can range from4 to 255 rather than from 6 to 257.

Partial virtual bitmap 712 may become unnecessarily long, especially ifthe number of stations having buffered data is relatively small makingthe partial virtual bitmap sparse. In an extreme example, supposebuffered data is awaiting two stations with AID 1 and AID 1001. In orderto indicate this, partial virtual bitmap 712 would have to be 126 octetsin length.

FIG. 10 shows an embodiment of a TIM element having two partial virtualbitmaps. Rather than have one partial virtual bitmap of 251 octets, twoshorter partial virtual bitmaps could be used. Using the extreme examplegiven above, traffic for the station with AID 1 would be representedwith a zero offset reflected in bitmap control 1010. Partial virtualbitmap 1012 would only need to be one octet wide. Traffic indication forthe station with AID 1001 would then be represented in the second set ofTIM fields by a bitmap control 1014 which would indicate an offset of125 octets and partial virtual bitmap 1016 which would only need to beone octet wide. A total of four octets would be used compared to 126.Clearly, this process could be repeated for more than two sets of TIMfields, when the stations with buffer data is sparse.

FIG. 11 shows a TIM management action frame format. Fields 1102, 1104,1106, 1108, 1110 and 1112 are often collectively referred to as themedia access control (MAC) header. More specifically, frame controlfield 1102 is similar to frame control field 802 in FIG. 8, in this caseindicating that the frame is an action frame. Duration/ID field 1104 issimilar to Duration/ID field 804 in FIG. 8. Destination address field1106 is similar to receiver address field 806 and should be set to thespecial broadcast address as specified by the particular standard.Source address field 1108 is a six octet fixed field, which isindicative of the source in this case, is set to the basic service setidentification (BSSID) because the source is the access point which hasthe BSSID as its MAC address. Address field 1110 is a six octet fixedfield which is indicative of the BSSID. Sequence control field 1112 is atwo octet fixed field which comprises a fragment number and a sequencenumber. The fragment number is used when a frame is fragmented to keeptrack of the fragments. The sequence number is incremented each time astation transmits a message. Category field 1114 is a one octet fieldindicative of the category of action in a management action frame. Inthis case, a TIM action frame would fall under the category of WirelessNetwork Management. Action field 1116 is a one octet field indicative ofthe specific action within the category. In this case, the action is aTIM frame. TIM element 1120 is similar to the TIM elements describedabove. It can be the 6 to 257 octet TIM element of FIG. 7 or the 4 to255 octet TIM element of FIG. 9. Finally, frame check sequence field1122, like frame check sequence 810 is a four octet fixed fieldindicative of the integrity of the frame.

While a TIM control frame as depicted in FIG. 8 is shorter and wouldrequire a station to awaken for a shorter period of time, control framestypically are implemented at a lower level and often would require achange in hardware to support it. In contrast, management frames and inparticular management action frames are intended to be extensible andthe number of actions tends to grow as standards evolve. Therefore, aTIM management action frame is easier to implement than a TIM controlframe.

Timing of a TIM frame is critical as a station in standby mode must knowwhen to wake up to look for the TIM frame. FIGS. 12A-C illustrate someexemplary timing schemes. As illustrated in FIG. 12A, in the firstscheme, TIM frames 1202 and 1206 are transmitted at a time which isequal to the estimated length of the TIM frame with a SIFS prior to theTBTT. Beacons 1204 and 1208 are transmitted at their respective TBTTs.The estimated length used could be the maximum possible length of theTIM which can range from 269 to 285 octets depending on the form of theTIM frame used. The estimated length could also be based on the minimumlength, or the average length of the TIM frame. Stations in standby modethat are aware of this TIM frame would then wake up at this time toreceive the TIM frame. The access point can announce the presence ofseparate periodically sent TIM frames in several ways as available inthe specific standard used. For example, an announcement element can beincluded in a beacon frame. Stations can also inquire as to propertiesof an access point by transmitting a probe request frame. In response toa probe request, an access point transmits a probe response whichcomprises many of the same parameter sets and informational elements asis present in the beacon frame. The announcement element described abovecan also be included in such a probe response.

FIG. 12B illustrates an alternate timing, where TIM frames 1232 and 1236are transmitted at the TBTT. In this example, beacon frames 1234 and1238 are postponed until after the transmission of TIM frames 1232 and1236, respectively, followed by a respective SIFS. Stations searchingfor a beacon frame would then have to wait the length of the TIM framefollowed by the SIFS to receive the beacon frame. A hybrid approach toFIG. 12A and FIG. 12B can also be implemented where the TIM frame istransmitted prior to the TBTT but not as early as described in FIG. 12A,so that the beacon frame is still postponed past the TBTT but by a timeinterval shorter than described in FIG. 12B. The interval prior to theTBTT could be announced through an announcement element in a beaconframe and/or probe response, so that a station in standby mode is awareof when to wake up to receive the TIM frame.

FIG. 12C illustrates a more general timing, where the TIM frame istransmitted any time between beacon frames. In this example, thetransmission of TIM frame 1264 follows the TBTT of beacon frame 1262 bytime offset 1266. The time offset from the TBTT for a TIM frame can beannounced through an announcement element in a beacon frame and/or proberesponse, so that a station in standby mode is aware of when to wake upto receive the TIM frame. For example, the announcement element couldcomprise a fixed field indicating the number of microseconds after aTBTT that a TIM frame will be transmitted. The offset could be negative,which indicates that the TIM frame is transmitted before the TBTT.

It is not necessary to transmit a TIM frame every beacon interval. Evenif there is no buffered data awaiting a station, a station in standbymode wakes up every DTIM beacon frame to receive buffered multicastdata. As described above a DTIM beacon frame occurs once every DTIMperiod. Therefore, a TIM frame could be broadcast relative to a DTIMTBTT, which is a TBTT associated with a DTIM beacon frame. As a result,TIM frames are transmitted less frequently, and a station in standbymode need not wake up as frequently, hence saving power.

FIG. 13A illustrates the timing where a TIM frame is transmitted priorto a DTIM TBTT similar to the example shown in FIG. 12A, except that aTIM frame is transmitted immediately prior to only the DTIM TBTT and notother TBTTs. Again, the timing specifics can be announced through anannouncement element in a beacon frame and/or probe response.Specifically referring to FIG. 13A, TIM frame 1302 is sent prior to DTIMbeacon frame 1304, but no TIM frame proceeds regular beacon frame 1306.

FIG. 13B illustrates the timing where a TIM frame is transmitted at aDTIM TBTT similar to the example shown in FIG. 12B. The TIM frame istransmitted only at the DTIM TBTT and not other TBTTs. As a result theDTIM beacon frames are postponed until after the TIM frame istransmitted. However, all other beacon frames are transmitted at theirrespective TBTTs. Specifically as illustrated, TIM frame 1352 istransmitted at the DTIM TBTT causing DTIM beacon frame 1354 to bedelayed until after TIM frame 1352 and an SIFS. However, regular beaconframe 1356 is transmitted at the TBTT. Like above, a hybrid approach toFIGS. 13A and 13B can used where the TIM frame is transmitted prior tothe DTIM TBTT, but not as early as in FIG. 13A. The result is that thetransmission of the DTIM beacon frame is postponed but by a factor lessthan that given in FIG. 13B. The transmission of other beacon frames isunaffected.

Like the example given in FIG. 13C, an arbitrary timing relative to theDTIM TBTT can be given for a TIM frame. The TIM frame time offset can beannounced through an announcement element in a beacon frame and/or proberesponse, so that a station in standby mode is aware of when to wake upto receive the TIM frame. For example, the announcement element couldcomprise a fixed field indicating the number of microseconds after aDTIM TBTT that a TIM frame will be transmitted.

To further reduce the amount of time a station in standby needs to stayawake, it is desirable for a TIM frame to send at a higher PHY rate.However, a TIM frame should be sent at the lowest PHY rate so that evenstations having the lowest quality connections can determine whetherthey have buffered data at the access point. To accommodate both ofthese conditions, two or more TIM frames can be transmitted at differentrates. If a station is able to received a TIM frame at a higher datarate, the receive time can be reduced significantly. For example, justfor the PHY header portion of the TIM frame, it would take 192 μs totransmit using the lowest PHY rate using DSSS, but it would only take 20μs using a higher PHY rate using orthogonal frequency divisionmultiplexing (OFDM). This is nearly an order of magnitude difference.Preferably, the TIM frames should be transmitted in order of data ratewith the highest rates being transmitted first, so that if a station isunable to receive the higher rate TIM frame, it still has theopportunity to receive the one transmitted at a lower rate. For clarity,two different rate TIM frames are shown, but it is understood theapproach can apply to more than two rates. For convenience, the TIMframe transmitted at the higher PHY rate will be referred to as the highrate TIM frame, and the TIM frame transmitted at the lower (or lowest)PHY rate will be referred to as the low rate TIM frame.

FIG. 14A illustrates one timing for transmission of two TIM frames atdifferent PHY rates. The schedule of the TIM frame can employ a knownoffset relative to TBTT which can be negative. This offset can bepredetermined or announced through an announcement element in a beaconframe or probe response. The offset may embody a negative or zerointerval so that the timing can resemble that of FIGS. 12A and 12B orany interval in between. For clarity, a positive offset is illustrated.

Specifically, FIG. 14A shows high rate TIM frame 1404 being transmittedat time offset 1408 after TBTT, the TBTT after which beacon frame 1402is transmitted. Immediately after an SIFS, low rate TIM frame 1406 istransmitted. A station capable of receiving a high rate TIM frame canwake up at time offset 1408 after the TBTT to receive high rate TIMframe 1404 and return to standby mode, if no buffered data is waiting atthe access point. A station where the capability is uncertain can wakeup at time offset 1408 after the TBTT to attempt to receive high rateTIM frame 1404. If the station is unable to receive high rate TIM frame1404, it then receives low rate TIM frame 1406. If the station is notcapable of receiving the high rate TIM frame, it can wake up at a timeequal to an interval, which is the sum of time offset 1408, the minimumpossible (or typical) transmission time of high rate TIM frame 1404, andan SIFS, after the TBTT; that is, it can wake up at the earliestpossible time low rate TIM frame 1406 can be transmitted. Once the TIMframe is received at whatever rate the station is capable of, thestation can return to standby mode if no buffered data is waiting at theaccess point. The stations incapable of receiving the high rate TIMframe may suffer a little penalty of having to remain awake a littlelonger to accommodate the inclusion of a high rate TIM frame. However,because the high rate TIM frame is transmitted at a high rate, thepenalty will be generally small.

FIG. 14B shows a timeline where both a high rate TIM frame and a lowrate TIM frame are individually scheduled. This timing eliminates theslight penalty mentioned above. Specifically, high rate TIM frame 1454is transmitted at high rate time offset 1458 after TBTT, the TBTT afterwhich beacon frame 1452 is transmitted. Low rate TIM frame 1456 istransmitted at basic time offset 1460 after the same TBTT. A stationcapable of receiving high rate TIM frame 1454 at high rate time offset1458 after TBTT to receive the TIM frame, whereas a station incapable ofreceiving high rate TIM frame 1454 will instead wake up at basic timeoffset 1460 after TBTT to receive low rate TIM frame 1456. A stationthat is uncertain of its capability can first wake up to receive highrate TIM frame 1454 and wake up again to receive low rate TIM frame1456, if it was unable to receive high rate TIM frame 1454.

One of ordinary skill in the art can appreciate that a combination ofthe methods shown in FIGS. 12A-C and 13A-B can be combined. The variouspermutations can be used. For example, the multiple TIM frames couldoccur only after DTIM TBTT, rather than every beacon interval or ahybrid timing where the high rate TIM frames occur only after DTIM TBTT,and low rate TIM frames occur after every TBTT, or vise versa.Furthermore, more than two rates could be employed as mentioned above.

The timing of FIG. 14A shows how multiple TIM frames transmitted atdifferent rates can be sent as a burst. A burst can also be used totransmit multiple TIM frames at the same rate. As mentioned above withregard to FIG. 10, multiple TIMs comprising different bitmap controlfields and partial virtual bitmap fields, can be used in place of a longTIM when the partial virtual bitmaps are long and sparse. Instead ofcreating a more complicated TIM element as illustrated in FIG. 10,multiple TIM frames containing a TIM element as illustrated in FIG. 9are sent as a TIM frame burst.

Specifically, FIG. 15 shows multiple TIM frames sent as a burst. Forclarity, a positive time offset relative to the TBTT is depicted, but anegative or zero offset can be used. A TIM frame burst is transmitted attime offset 1508 after TBTT, the TBTT after which beacon frame 1502 istransmitted. The TIM frame burst comprises TIM frame 1504 and 1506 whichcan contain different bitmap control fields and different partialvirtual bitmap fields. The two TIM frames are separated by an SIFS. Astation after receiving TIM frame 1504 can be aware that TIM frame 1506is part of the current burst by using standard burst indications. Forexample, in 802.11, the frame control field, such as frame control field802 in FIG. 8 and frame control field 1102 in FIG. 11, comprise a moredata subfield which is set for all frames in a burst, except for thelast frame. Of course, a station need not continue to receive TIM framesafter it has received the TIM frame, which has an indication about theparticular station's AID, that is, if the partial virtual bitmap has abit allocated to represent the stations AID whether set or not, thestation can then disregard all subsequent TIM frames in the burst.

Furthermore, it is understood that though the number of frames depictedis two, more than two frames can be present in the burst. The number offrames in the burst can vary from beacon interval to beacon intervaldepending on how sparse the TIM is at each beacon interval. This can becombined with the multiple rate TIM transmissions by transmitting TIMframe bursts at multiple rates. The bursts need not be transmitted everybeacon interval and can be transmitted only at the DTIM beaconintervals.

As described in the background section, there may be information withina beacon frame that an associated station, even one in standby mode,needs to retrieve. For example, the beacon frame may contain a channelswitch announcement, which indicates that the BSS will move to anotherchannel shortly. In another example, the beacon frame may indicate thatthe access point changes the Enhanced Distributed Channel Access (EDCA)parameters. However, if a station in standby mode has to wake up toreceive each beacon frame to receive potential changes in the beaconframe, having a separate TIM frame derives no benefit. The power savingsderives from the station only receiving the much shorter TIM frame.

To address this potential difficulty, the TIM frame can also include acheck beacon field. FIG. 16 illustrates a TIM control frame, whichincludes check beacon field 1608, which is a one octet fixed field. Allremaining fields and elements are similar to their correspondingcounterparts in FIG. 8. Check beacon field 1608 is indicative of whethera change in the beacon frame has occurred that is significant andwarrants a station to read the following beacon frame, such as examplesgiven above. Insignificant changes such as changes to the timestamp arenot indicated by this field. Check beacon field 1608 could simply be aBoolean state which indicates whether the following beacon frame haschanged significantly relative to the past beacon frame. However, if astation somehow missed the TIM frame indicating the change in thebeacon, it may never become aware that a change has occurred. Anotherapproach is that check beacon field 1608 is a counter which isincremented modulo 255 whenever a beacon frame has changed significantlyrelative to the past beacon frame. A station receiving a TIM framecompares (modulo 255) the value of check beacon field 1608 relative tothe value of check beacon field in a previously received TIM frame. Ifthe current value is greater (modulo 255) than the previous value, thestation should receive the next beacon frame. Otherwise, the stationneed not stay awake for the next beacon frame and may elect to go intostandby mode.

The changes in the beacon frame could be categorized as significant andcritical where a significant but not critical change would not requirethe station to receive a beacon immediately but in the near future, anda significant and critical change would require the station to receive abeacon frame immediately. For example, a change in the EDCA parametersis significant, where failing to receive them affects quality ofservice. However, a channel switch announcement is critical, sincefailing to receive it would result in the station losing communicationswith the access point. In another embodiment, the eight bits of checkbeacon field 1608 could be divided into two counters, preferably a 3-bitcritical change counter and a 5-bit significant change counter(presumably critical changes occur less frequently) or a 4-bit criticalchange counter and a 4-bit significant change counter. Alternatively,check beacon field 1608 could be expanded to two octets where each octetrepresents an 8-bit counter that are incremented for each criticalchange or each significant change respectively.

FIG. 17 illustrates the alternative embodiment of a TIM frame with acheck beacon indication where the TIM frame is a management actionframe. Check beacon field 1718 can take on any of the embodimentsdescribed for FIG. 16. The actions of the station upon receiving checkbeacon field 1718 is the same as above. All remaining fields andelements are similar to their corresponding counterparts in FIG. 11. Theadvantages and disadvantages of the use of a TIM management action frameover a TIM control frame are discussed above.

FIG. 18 is a flowchart showing exemplary logic which can be implementedin the software of a station showing the interoperation of the receivingof the TIM, the check beacon indication, and receiving of the beacon. Atstep 1802, the station wakes up. This should be the time set forth byone of the timing approaches previously discussed for the expectation ofa TIM frame. At step 1804, the station receives the TIM frame. Based onthe TIM frame, at step 1806, there is a determination made as to whetherthere is buffered data waiting at the access point. If there is buffereddata, the station may elect to remain awake and receive the next beaconframe. This is optional, but since the station may stay awake toretrieve the data, it may also receive the beacon frame. It may alsodecide to receive the beacon frame because a previous significant changehas occurred, but the station elected not to receive the beacon at thetime the change was detected. At step 1810, the station can retrieve thedata by using a PS-Poll message sequence. At step 1820, the station canreturn to standby mode. On the other hand, if no buffered data iswaiting for the station at the access point, as determined in step 1806,the station checks the check beacon indication for the occurrence of acritical beacon change at step 1812. If there is a critical beaconchange, the next beacon frame is received at step 1818, and the stationcan return to standby mode at step 1820. If no critical beacon changehas occurred, the station then can check the check beacon indication forthe occurrence of a significant beacon change at step 1814. If there isno significant beacon change the station can return to standby withoutreceiving the beacon at step 1820. If there is a significant beaconchange, the station determines whether it should receive the beacon atstep 1816. There are many possibilities for the determination at thisstep. For example, the station may only receive the beacon when there isbuffered data, so it may defer receiving the beacon until such time thedecision at step 1808 receives an affirmative decision. In othercircumstances, the station may wait a certain period of time beforerequiring the beacon to be received or a combination of the previous twosituations. In still another circumstance, the station does notrecognize significant but not critical changes in the beacon, so it maynever determine to receive the beacon at this step. If the decision ismade not to receive the beacon, the station can return to standby atstep 1820. Depending on the implementation, it may record that thebeacon has had a significant change before returning to standby. If thedetermination at step 1816 is to receive the beacon, the stationreceives the next beacon frame at step 1818.

The flowchart is expressed in general with respect to the use of TIMframes. However, this can be used with the remainder of the partialbeacon receiving approaches disclosed below. In particular, at step1804, the first part of the beacon frame is received up to at least thepoint where the TIM element or ETIM element is received. At steps 1808and 1818, the station receives the remainder of the beacon; otherwise,if the path flows from step 1814 to step 1820, the station can return tostandby without receiving the remainder of the beacon.

While FIG. 6 illustrates the first 13 elements in a beacon frame inaccordance with an exemplary wireless protocol. The order of thesefields is suggested by the standard. However, only the first threefields are fixed fields and cannot be rearranged. The remaining fieldsare informational elements and can be identified by the element ID fieldwithin each element and therefore can be rearranged. A TIM element canbe included near the beginning of the beacon frame so that a station instandby mode need only wake up to receive part of the beacon frame todetermine whether the access point has buffered data waiting for it.Since all informational elements are identified by the element ID fieldwithin each element, including the TIM element after the three fixedfields, the beacon frame should be interoperable with legacy systems.

Moving the TIM element after the three fixed fields would enable astation in standby mode to receive only part of the beacon frame;however, the same issue applies as discussed above that in certaincircumstances a station in standby mode should receive the entirebeacon. A check beacon informational element can be included in thebeacon frame to indicate when significant changes to the beacon frameoccur.

FIG. 19 illustrates the first portion of a beacon frame in accordance toone embodiment of the invention. Fields 1902, 1904, 1906, 1908, 1910,and 1912 are part of the standard MAC header, similar to that describedfor FIG. 11 This is followed by timestamp field 1914, beacon intervalfield 1916, and capability information field 1918, the three requiredfixed fields in a beacon. Timestamp field 1914 is an eight octet fixedfield comprising the time the present frame is sent. Beacon intervalfield 1916 represents the number of time units between TBTTs. Capabilityinformation field 1918 contains a number of subfields that are used toindicate requested or advertised capabilities. Following the three fixedfields, the beacon includes check beacon informational element 1920,which is described in further detail below, and TIM element 1922 whichis the standard TIM element as describe above in FIG. 7. Equivalently,TIM element 1922 could precede check beacon informational element 1920.

FIG. 20 illustrates an exemplary embodiment of the check beaconinformational element. The exemplary check beacon information elementcomprises two octet element ID fixed field 2002 and two octet lengthfield 2004 which are standard in any informational element. Element IDfield 2002 contains a new information element identifier that isassociated with the check beacon informational element. It alsocomprises check beacon field 2006 which is indicative of significantand/or critical change in the beacon. The manner of indication couldembody any of the variations described for FIG. 16.

Using an embodiment of the check beacon field that is one octet and thesmallest possible TIM element size, the portion of the beacon that mustbe read to determine whether the access point has buffered data waitingis 45 octets. At 1 Mb/s direct sequence spread spectrum rate, receivingthis portion of the beacon frame takes 552 μs. which is considerablyshorter that the typical 2 ms a complete beacon frame would currentlytake to receive. The fact that a station need only be awake a fractionof the time to determine whether the access point has buffered datawaiting can conserve power in the station.

One difficulty in ignoring the remainder of the beacon frame is that thelast field in any frame is the frame check sequence (FCS) which is usedto determine the integrity of the received frame. Without receiving theFCS, the station cannot be sure whether the received portion is correct.If the TIM element is corrupted, the consequences are slight. An errorcould cause the station to wake up and poll for buffered data when thereis none, in which case the station will discover there is no data andreturn to standby. Alternatively, an error could cause the station tostay dormant when the access point has buffered data waiting, in whichcase delivery will be delayed until the next beacon frame. The lattercase would occur without a partial beacon frame reception. Had thestation received the entire beacon, it would have discarded the beaconframe because the FCS would have indicated the beacon frame was corrupt.

In the event the check beacon field is corrupt, the receiver might readthe entire beacon frame when it didn't need to. If the station isconfigured to read the entire beacon if the value in the check beaconfield differs from that received in the previous frame, rather thansimply greater than received in the previous frame, the chance of acorrupt check beacon field causing a partial reception of the beaconframe when a full reception is warranted diminishes. Despite theconsequences being slight, if the check beacon field is corruptedfrequently, such as when the conditional error rate in the TIM and checkbeacon element increase, the power consumption would increase due to thestation having to unnecessarily wake up. The addition of a partial framecheck information element could eliminate these issues.

FIG. 21 illustrates a partial frame check informational elementexemplary embodiment. As with all information elements, element ID field2102 and length field 2104 are present and similar to that describedabove for other elements. Element ID field 2102 contains a newinformation element identifier that is associated with the partial framecheck informational element. The partial frame check informationalelement would also comprise a partial frame FCS which could simply bethe CRC or even a 1-bit parity check of the check beacon informationelement and the TIM element. It could also include the timestamp field,the beacon interval field and the capability information field. Fieldsin the MAC header do not need to be considered otherwise the frame wouldnot be recognized as a valid beacon frame. The partial frame checkinformation element could be inserted into the beacon right after TIMelement 1922.

Rather than create two new informational elements, the threeinformational elements could be combined into an ETIM element. FIG. 22shows an exemplary first portion of the beacon frame using ETIM element.Fields 2202, 2204, 2206, 2208, 2210, 2212, 2214, 2216 and 2218 aresimilar to their counterparts, fields 1902, 1904, 1906, 1908, 1910,1912, 1914, 1916 and 1918, respectively, as described above for FIG. 19.Rather than including a TIM element along with a check beaconinformational element and potentially a partial frame check element,ETIM element 2220 is included.

FIG. 23 shows the format of an ETIM informational element. As with allinformational elements, element ID field 2302 and length field 2304 arepresent and similar to that described above for other elements. ElementID field 2302 contains a new informational element that is associatedwith ETIM informational elements. Check beacon field 2306 is a one octetfield indicative of whether the beacon frame contains significant and/orcritical changes. The manner of indication could embody any of thevariations described for FIG. 16. Bitmap control field 2308 is similarto field 710 that is described in FIG. 7. Partial virtual bitmap field2310 is similar to field 712 that is described in FIG. 7. Collectively,bitmap control field 2308 and partial virtual bitmap field 2310 arereferred to as TIM fields. Finally, error detection field 2312 providessome sort of integrity check of check beacon field 2306 and the TIMfields such as a 1-bit parity check or a more complex CRC of multiplebits. Other fields in a TIM informational element such as the DTIM countfield and the DTIM interval field could be present in the TIM fields butare not required.

For similar reasons as explained for FIGS. 13A and 13B, the ETIMinformational element need not be included in every beacon. They may,for example, only be included in DTIM beacons. In this manner, a stationin standby mode need only wake up during DTIM beacons to determine ifthe access point has buffered data waiting. The ETIM informationalelement could alternatively be provided at ETIM interval that is every nbeacons where n is the ETIM interval. This could be negotiated through astartup mechanism. The ETIM interval could also be announced by theaccess point in a beacon frame or a probe response.

It should be emphasized that the above-described embodiments are merelyexamples of possible implementations. Many variations and modificationsmay be made to the above-described embodiments without departing fromthe principles of the present disclosure. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and protected by the following claims.

1. An informational element comprising: a check beacon indication field;a bitmap control field; and a partial virtual bitmap field.
 2. Theinformational element of claim 1, further comprising an error detectionfield.
 3. The informational element of claim 1, wherein the check beaconindication field comprises means for indicating a critical change in thebeacon frame.
 4. The informational element of claim 1, wherein the checkbeacon indication field comprises a counter which is incremented aftereach critical change in the beacon frame.
 5. The informational elementof claim 1, wherein the check beacon indication field comprises meansfor indicating a significant change in the beacon frame.
 6. Theinformational element of claim 1, wherein the check beacon indicationfield comprises a counter which is incremented after each significantchange in the beacon frame.
 7. The informational element of claim 1,wherein the error detection field comprises a parity check.
 8. Theinformational element of claim 1, wherein the error detection fieldcomprises a cyclic redundancy code (CRC).
 9. The informational elementof claim 1, wherein the error detection field comprises a hash.
 10. Theinformational element of claim 1, further comprising a second bitmapcontrol field and a second partial virtual bitmap.
 11. A method forindicating buffered data comprising, broadcasting a beacon framecomprising the informational element of claim 1, wherein theinformational element is placed near the beginning of the beacon frame.12. The method of claim 11, wherein the beacon frame is a deliverytraffic indication map (DTIM) beacon frame.
 13. A method in a stationfor retrieving buffered data from an access point comprising: partiallyreceiving a beacon frame, said partially receiving comprising receivingan embedded traffic indication map (ETIM) informational element, whereinthe ETIM informational element comprises: a check beacon indicationfield; a bitmap control field; and a partial virtual bitmap field. ifthe partial virtual bitmap is indicative that the access point hasbuffered data for the station, receiving the buffered data; and if thecheck beacon indication field indicates a critical or significant changein the beacon frame, receiving the remainder of the beacon frame. 14.The method of claim 13, wherein the ETIM informational element furthercomprises an error detection field.
 15. The method of claim 13, whereinthe check beacon indication field comprises a means for indicating acritical change in the beacon frame, a means for indicating asignificant change in the beacon frame, or both.
 16. The method of claim13, wherein the check beacon indication field comprises a first counterwhich is incremented after each critical change in the beacon frame, asecond counter which is incremented after each significant change in thebeacon frame, or both.
 17. The method of claim 13, wherein the errordetection field comprises a parity check, a CRC, a hash or a combinationthereof.
 18. A station comprising a processor, a wireless networkinterface device and a memory comprising instructions, said instructionscausing the processor and the wireless network interface device toperform the method of claim
 13. 19. The method of claim 14, furthercomprising discarding the partially received beacon frame if the errordetection field indicates an error.
 20. A station comprising aprocessor, a wireless network interface device and a memory comprisinginstructions, said instructions causing the processor and the wirelessnetwork interface device to perform the method of claim
 19. 21. Anaccess point comprising: a processor; a wireless network interfacedevice; and a memory comprising instructions; said instructions causingthe processor to cause the wireless network interface device tobroadcast a beacon frame comprising an ETIM element, wherein the ETIMelement is placed near the beginning of the beacon frame; said ETIMelement comprising: a check beacon indication field; a bitmap controlfield; and a partial virtual bitmap field.
 22. The access point of claim21, wherein the ETIM element further comprises an error detection field.23. The access point of claim 21, wherein the ETIM element immediatelyfollows a capability field in the beacon frame
 24. The access point ofclaim 21, wherein the ETIM element comprises a second bitmap controlfield and a second partial virtual bitmap.
 25. The access point of claim21, wherein the check beacon indication field comprises a means forindicating a critical change in the beacon frame, a means for indicatinga significant change in the beacon frame, or both.
 26. The access pointof claim 21, wherein the check beacon indication field comprises a firstcounter which is incremented after each critical change in the beaconframe, a second counter which is incremented after each significantchange in the beacon frame, or both.
 27. The access point of claim 21,wherein the error detection field comprises a parity check, a CRC, ahash or a combination thereof.
 28. The method of claim 21, wherein thebeacon frame is a DTIM beacon frame.